FIG. 6 shows a schematic configuration of an atomic absorption spectrophotometer most commonly used. The atomic absorption spectrophotometer includes a hollow cathode lamp having a cathode made from an objective element as a light source 10, whereby a light containing a line spectrum of the resonance lines of the objective element is generated. When the light generated from the light source 10 passes an atomized sample in an atomization unit 11, part of the light at the wavelengths of the resonance lines are selectively absorbed more than the other part of light. After passing the sample, the light is dispersed and selected by a monochromator 12, whereby a monochromatic light having a wavelength of one of the resonance lines of the objective element is extracted. When the monochromatic light is received by a photodetector 13, the photodetector 13 sends a detection signal to an amplifier 14, and the signal amplified thereby is sent to a signal processing unit 15. In the signal processing unit 15, the data on the loss of the intensity of light from the light source is processed to obtain an absorption spectrum within a range including the resonance lines, and the absorption spectrum is displayed by an output unit 16 such as a display.
The monochromator 12 includes an entrance slit, a diffraction grating, an exit slit, etc. The diffraction grating can be rotated by a motor and a monochromatic light of a desired wavelength .lambda. can be separated by changing the orientation of the diffraction grating. A speed reducing mechanism is provided between the motor and the diffraction grating because it is necessary to rotate the diffraction grating by a very small angular step when the scanning of the wavelength must be carried out by a very small wavelength interval.
When a pulse motor is used to rotate the diffraction grating, the orientation of the diffraction grating is controlled by, for example, the following steps. First, the relation between the number of the driving pulses, which is the controlling input of the pulse motor, and the wavelength of the light to be extracted by the monochromator 12 is analyzed beforehand, and the result is stored in the form of a table in a memory device. After that, when an analysis for an element is carried out, the numbers of the driving pulse corresponding to the wavelength of resonance line of the element is determined by referring to the table in the memory and the pulses are sent to the pulse motor by the above number, whereby the diffraction grating is set at an angular position corresponding to the above wavelength. Thus a monochromatic light having the wavelength of a resonance line of the objective element is extracted by the monochromator 12.
In the above system, however, usually a transmission error occurs in the relation between the number of driving pulses of the pulse motor and the orientation of the diffraction grating due to the accumulation of minute errors in the parts constituting the speed reducing mechanism. FIG. 7 is a graph showing an example of the transmission error with respect to the feed angle of the motor. When such a transmission error exists, the wavelength of the monochromatic light separated by the monochromator 12 is displaced from the objective wavelength. Here, the degree of the displacement differs depending on the system being used since each system has its own inherent transmission error. In view of this, an expensive speed reduction mechanism including very precise parts such as a harmonic driving mechanism, ball threads, ground feed threads, etc., is used in the conventional system in order to suppress the transmission error mechanically so that the displacement of the wavelength is minimized.
In view of the above problem, the present invention proposes an atomic absorption spectrophotometer which can be used for an analysis with high accuracy even without an expensive speed reducing mechanism.